FRESNO, Calif. — Malfunctioning heating systems in some school district schools in Fresno, Calif., have forced students and teachers to bundle up in winter coats and heavy sweaters as they sit in chilly classrooms.

At least four schools have had ongoing problems with heaters over the past several weeks, according to the Fresno Teachers Association, which has fielded numerous complaints from teachers.

The teachers union even issued thermometers to about 10 teachers Wednesday, hoping to record how cold their classrooms get.

"When teachers come in in the morning, the classrooms are freezing," FTA President Greg Gadams said. "This is a constant problem, and the district is slow to react."

But district officials say that as soon as they learned that the heat wasn't working at Malloch, Norseman and Hamilton elementary schools, maintenance workers fixed the problems, and by Tuesday the schools were warm again.

Only one school — Bullard High School — still doesn't have heat in roughly 24 classrooms because heating and air conditioning units are being replaced as part of a modernization project, Fresno Unified spokeswoman Susan Bedi said. Maintenance workers are trying to restore warmth as soon as possible, she said.

"This is a high priority," she said. Maintenance and operations workers "have delivered portable heaters to every classroom, and the teachers have been told to keep them on so that the classrooms are at a comfortable temperature."

Bullard teacher Sophia Smith said it was only about 40 degrees in her classroom Wednesday. Smith said she is a cancer survivor, and when it gets really cold, her body can spasm. She said she's had a difficult time speaking because her face has tightened up in her cold classroom.

Smith said the heat has never worked and she's been told it won't be fixed until January — after she returns from winter break.

"I'm wearing a wrap around my neck and doing everything I can to get warm," said Smith, who has taught at Bullard for 11 years.

An e-mail sent to the school's staff Tuesday by Bullard's office manager, Sally Barrera, said that boilers need to be replaced and the heat will be restored during winter break.

Barrera went on to say: "Talk about wishing for your two front teeth. Bundle up, folks."

Bedi clarified Barrera's e-mail by saying that at least one of the boilers has been replaced, but it's unknown when the other two will be. That's why, she said, the district supplied portable heaters to each of the affected classrooms.

As of late Wednesday, 17 of the 24 classrooms had working space heaters.

But Bullard teacher George Dugal said the school's wiring is not equipped to handle so many space heaters and fuses are being blown.

He also said the district should have replaced the heating system before the winter.

"It's almost impossible to think they would allow children to go into a classroom, in this kind of weather, that's not heated," he said. "And what happens if we come back in January and still have no heat? That is a very real possibility."